There are two types of heart fibrillation, namely, ventricular and atrial fibrillation.
When a person's heart is in ventricular fibrillation, death is imminent. In ventricular fibrillation, the heart cells that stimulate the heart muscles are not coordinated so that although they stimulate the muscles, they produce rapid, erratic excitation without coordinated contraction of the ventricle. There is no effective simultaneous action to cause the heart to beat in rhythmic fashion. To avoid death, immediate defibrillation is essential.
When a person's heart is in atrial fibrillation, the atria are uncoordinated and are not beating in a rhythmic fashion. Atrial fibrillation generally is not life threatening. Atrial fibrillation with a clinically stable patient may be resolved with drug therapy, and when necessary, with electrical fibrillation. If atrial defibrillation is coupled with rapid ventricular response and other medical problems, it may be necessary to perform emergency electrical defibrillation.
Defibrillation is achieved by delivering to the heart cells enough voltage to override the erratic voltages in the fibrillating heart so that they can rearrange themselves with order. That action is called "repolarization". In this "repolarized" condition no heart action of any kind occurs for a period of three to eight seconds. After this three to eight seconds period, the heart cells arrange themselves to either fibrillate again or start a coordinated effort to beat in rhythm. If they fibrillate again, it is necessary to defibrillate them again, usually with more power. It may be necessary to do this several times before the heart cells arrange themselves to coordinate and beat in rhythm.
Defibrillation by high power is the accepted procedure today. Heavy duty equipment is required to deliver a very severe electric shock to the patient. The shock is delivered by placing two large paddle-type electrodes, each about three inches in diameter, at selected locations on the chest. By pressing down hard on the electrodes to make good electrical contact with the skin, and by pressing a button provided on one of the paddle electrodes, the system is triggered so as to deliver the shock. The electrical shock is very abrupt; several thousand volts are impressed across the electrodes in a few milliseconds. In accordance with this procedure, in theory if the patient is given a large enough shock, some of it will pass though the heart and achieve repolarization. However, all the other muscles and nerves in the large area between the electrodes are also stimulated by the intense electrical shock, causing tremendous body flailing and thrashing. The power required for this type of defibrillation is a minimum of 100 joules; sometimes 200 joules are required, and usually 360 joules are necessary.